Garden news: Rose disease moving into Louisiana

LSU experts are warning Louisiana gardeners about rose rosette disease, which is a threat to roses in Louisiana.

All rose enthusiasts should know about this problem and look for symptoms, says LSU AgCenter “plant doctor” Raj Singh.

Caused by rose rosette virus, the disease was recently identified for the first time in Louisiana on landscape shrub roses growing in a commercial landscape in Bossier City.

Rose rosette disease produces a variety of symptoms, depending on the variety or species of the rose and the plant’s age. Some of the more recognizable symptoms include “witch’s broom,” excessive thorniness, thickened new canes and abnormal discoloration or excessive reddening of new foliage, Singh says.

Infected roses produce a cluster of new shoots from a single point on the parent canes and appear like a witch’s broom.

Infected canes also may produce excessive thorns that are green or red and soft in the beginning but later harden off as the disease progresses. This is one of the most reliable symptoms to help with diagnosing the disease.

Other symptoms are infected canes that are thicker than parent canes and reddening of new foliage and shoots.

“Remember, these symptoms can be used to potentially recognize rose rosette disease, but positive confirmation of the disease requires testing,” Singh says.

Improper use of herbicides, such as glyphosate, may result in distortion and clustering of new growth that looks like witch’s broom. Feeding injury from chilli thrips, which is a very significant rose-growing issue in Louisiana, also causes abnormal discoloration and distortion of new foliage.

Similarly, excessive reddening of new growth is a normal characteristic of some rose varieties. “There is no effective treatment, but we need to confirm the disease before anyone removes any plants,” Singh says. “Other things may cause similar symptoms.”

“Modern roses, which include our hybrid tea, grandiflora, floribunda and landscape shrub cultivars along with antique old garden roses are equally susceptible to the disease,” says LSU AgCenter horticulturist Allen Owings. “Naturalized plantings of the wild species of rose called multiflora are serving as host plants around the country.”

Gardeners who suspect they have rose rosette disease can email Singh at rsingh@agcenter.lsu.edu or call the AgCenter Plant Disease Clinic at (225) 578-4562. They also may take pictures of symptomatic plants and send them by email.

More information about avoiding and reducing the spread of the disease is available online at lsuagcenter.com by putting Publication 3355 in the search box.